1. Field of the Invention
The present invention relates to turbidimeters, and more particularly to apparatus for detecting particle concentrations in fluids using reflected radiation, such as infrared radiation.
2. Description of the Prior Art
The prior art contains optical sensors and submersible electronics units employed for measuring particle concentration in oceans, rivers, laboratories, or industrial applications. One type of prior art optical sensor has an infrared emitting diode (IRED) which generates infrared radiation with a peak radiant intensity at 950 nm, and includes radiation sensing cells such as solar cells for detecting the amount of infrared radiation reflected, or backscattered through angles from 110.degree. to 165.degree., by particles. A WRATTEN filter with transmittances of 0.55 percent at optical wavelengths less than 790 nm and 83.2 percent at optical (infrared) wavelengths above 950 nm, is superimposed over the sensing cells to eliminate visible radiation while permitting infrared radiation to pass to the sensing cells. Infrared radiation is partially absorbed by fluids such as water to thus limit the measurement volume (typically 3 cc) and render the sensor insensitive to reflective surfaces and infrared sources outside the measurement volume. Additionally, infrared radiation is weakly reflected from gas bubbles and translucent biogenic material such as phytoplankton at large backscatter angles. The infrared emitter is energized with an alternating current signal, such as a 1.024 kHz square wave signal, and the detecting circuitry employs a synchronous detector to detect the magnitude of the alternating signal in the reflected radiation to eliminate sensing of background radiation.
The prior art turbidimeter sensors are generally characterized by one or mor deficiencies such as being excessively expensive and costly to produce, being subject to corrosion and failure, having insufficient range and sensitivity to detect a wide range of particle concentrations, being subject to variation due to temperature variation of the fluid being tested, being responsive to radiation scattered at smaller angles, being subject to component drift, being relatively non-linear in response, having excessive variations resulting from manufacture, etc.